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How To Bypass the Factory Amps (step by step)


Slartibartfast
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I finally got around to doing this last night, and was pleasantly surprised by how easy it was. I took a few pictures of the job and figured I'd share them with the class. Most of the info came from the site linked below. Major thanks to the member who posted that link a few weeks ago!

 

Bypassing the Factory Amps

 

The factory head unit doesn't drive speakers on its own. Its output is line-level, and goes to the two Clarion amps that drive the actual speakers. Put an aftermaket head unit its place and the amps become unhelpful. You can't turn up the head unit much or you'll blow your eardrums out, so you end up sending a quiet signal to an amp that's been sitting in a quarter panel for 18+ years. I lived with this setup for a year or two, and it wasn't terrible, but after reading about other guys' stereos I decided to see if my head unit would sound better on its own.

 

Note that this isn't as good as running new and proper speaker wires. The factory wires are quite small and their run is pretty convoluted. If you've just dropped a couple hundred bucks on speakers and a nice head unit, you might as well wire it up properly. If you're like me with a Sony Xplod, $30 speakers in the doors, and factory Clarions everywhere else, this is pretty good bang-for-your-buck.

 

You will need...

 

This Crutchfield table for your model year, printed out (links to other years are at the bottom).

Ratchet with a 14mm socket

Big phillips screwdriver

Small slotted screwdriver

Wire cutters

Wire strippers

Solder

Shrink wrap

Soldering iron or butane torch

Two capacitors. I used 4.7μF electrolytic caps after seeing them recommended on Nico for use on R50 tweeters. Make sure they're rated for more than 12v. Mine are 35v and cost $4 for the two at a Radioshack. (If you don't have tweeters, you don't need these.)

 

My truck has the eight-speaker system. Six-speaker hookup is the same. Four speaker will be simpler (no tweeters). The '95 wiring table has separate sections depending on how many speakers you have, so make sure you're looking at the right one.

 

The amps in 94-95 are located in the right hand side of the cargo area. According to the Crutchfield tables, 93s have one of the two under the dash, and 87-92 have one amp, rear speakers only, in the dash. Since I did this writeup on a 95, some parts of it won't apply to older rigs.

 

A few plastic panels have to come out to access the amps. First, remove the strip of trim between the rear carpet and the hatch seal. This is the bit with the cutout for the hatch latch. The screws are plastic, so unscrew them carefully and without applying much downward pressure, or they'll skip right back into their holes. :headwall: Once they're backed out, the whole thing should more or less lift out. With that out of the way, fold down the passenger's rear seat, remove the access panel (the bit you have to reach into to get at the tail light), grab around the inside of the hole, and pull the whole plastic lower panel off. You'll have to bunch up the seatbelt a little to free it from its slot.

 

Now the upper panel has to come out. Push the plastic seat belt bolt cover out of the way and use the 14mm socket to remove the bolt that holds the hanger in place.

P1010013_zps09a1c625.jpg

 

Once that's out, find the plastic screw cover at the back corner of the trim piece. It's got a little cutout on one side where a slotted screwdriver can easily pop it out. Remove the phillips screw hiding behind it.

 

P1010016_zps13fc366c.jpg

 

The top panel should unsnap much like the lower one.

 

If your truck's an automatic, the passenger's rear quarter panel has three main units in it. The foremost one is the transmission computer, and it's in the way of the front amp. Undo its three screws, push it forward until the back of it can come out of the panel, then pull it out and set it aside.

 

P1010019_zpsdd651901.jpg

 

Remove the screws holding the rear amplifier bracket to the body. The aluminum-faced bit is the amp. It shares a bracket with a clear-cased electrical component. I'm not sure what the other bit does, but don't trash it, it isn't part of the amp.

 

P1010020_zpse6d14d0a.jpg

 

Remove this unit from its bracket by sticking a screwdriver under the metal strip that secures it and pulling it free. This reveals a couple screws holding the amp to the bracket; remove the amp and put the clear-cased bit back on. Put the bracket back where it was.

 

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Now pull out the front amp. It's a bit tricky to get the attachment points through the hole, but IIRC I pushed it back a ways and then pulled it out through the large hole where the trans computer sits. You can put the trans computer back now. Unplug both amps. (The small flat-bladed screwdriver will help with this.) Don't cut the plugs off just yet.

 

P1010024_zps5a5abdbb.jpg

 

The tables include color codes, but I found it much easier to sort out the wires while they were still in the plug. The diagram reads looking into the end of the plug BTW. The Crutchfield table tells you which wires are which and, at least in the 95 table, which ones to splice. All you're doing is hooking inputs to outputs. Left Rear + Input is going to go to the positive wire on the left rear speaker (Left Rear Speaker +), and so on. My only change to this setup is the wiring for the front tweeters.

 

For anyone not familiar with tweeters: they're just little speakers built for high frequencies. They're not built to handle bass, so we need to filter that out. Capacitors let current through until they've charged up, which makes them block constant direct current (DC) but pass alternating current (AC) or DC that fluctuates. Speaker signal is fluctuating DC with different frequencies. Because little tiny caps charge very quickly, low-frequency fluctuation is close enough to constant DC to get blocked, while higher frequencies go right through. I assume the factory amps cut filtered the frequencies going to the tweeter leads, and to run without them, we need to get some caps up in there. (The rear tweeters don't have their own wires from the amp, so whatever caps they've got between them and the rear speakers should continue to work with the new wiring job.)

 

Basically, you hook the tweeters up like normal speakers, except that one wire connects through a capacitor.

 

I tested my setup with jumper wires stuck into the end of the stock plug. It's not necessary but it is an easy way to make sure you've got your connections right before hacking into the harness.

 

P1010028_zps3de8aa76.jpg

 

The front plug has +/- input for either side, +/- for both door speakers (marked as woofers in the table), and +/- for each tweeter. That's three + leads and three - leads per side. The table shows where in the plug they are. Connect the three right + lines (input +, woofer +, tweeter +) together, then do the same for the left + lines. You'll have two joints, one for each side, both with three + wires. Strip, twist, solder, and insulate. (If you don't have shrink wrap small enough, put a chunk of a hot glue stick in there with the shrink wrap. It works great even if the shrink wrap is oversize.)

 

Now cut and strip the - leads for one side. Twist together the input -, the woofer -, and the - leg of a capacitor (marked by a black stripe on mine). Twist together the cap's + leg and the tweeter's + lead. Take care not apply heat directly to the capacitor or hold heat on its legs for too long. Insulate, then do the other side. You'll be left with two wires (12v accessory and amp remote) that go nowhere. Turn on the stereo (you may have to turn it up a bit) and check that sound is coming from all forward speakers. If it all works, wrap some electrical tape around the spliced end of the harness and ziptie it to one of the unused screw holes inside the panel to keep it from flopping around inside the panel.

 

If you're taking the front trim out anyway, or replacing your tweeters, you could just as easily mount the cap next to the tweeter. I was soldering the amp wires anyway, so this location made sense.

 

The rear plug goes just like the table says. Match like with like, strip, twist, solder, insulate. You'll make four connections and again be left with two unused wires. Turn on the stereo and make sure the rear speakers are working. Wrap and ziptie as with the other lead.

 

Once you're satisfied that it all works, put the panels back in. You may have to pop a few of the plastic clips out of the sheet metal and reinstall them in the panel before clipping it back on. Be careful installing the upper panel, as part of it has to slide in behind the panel that comes over the wheel well. If you don't align this before clipping it in and try to bend it into place, you'll kink the edge of it. :rolleyes: Once that's lined up, pop it back in place. Bolt the seatbelt hanger back up and slide the cover up over the bolt. Reinstall the phillips screw and its cover. Thread the seatbelt back into the lower panel, reinstall the panel, then put the door back in. Put the rear strip back in, make sure the clips are seated, and then reinstall the plastic screws. You can just push them in, but this may make them more difficult to remove next time.

 

That's about it. Power up your stereo and see what your head unit can do. Mine seems to have picked up a lot of bass, and it feels more balanced (I can turn it up louder without harshness). I turned on the 'loud' setting and it got the doors rattling. :lol: For $4 I call that an upgrade.

 

I know this isn't the most involved how-to ever but hopefully it'll help someone. :)

Edited by Slartibartfast
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My buddy has been having issues with his 93. The amp in the back (why does the back amp power the front, and the amp under the dash power the back?!?!?) has been cutting out on him so I think we'll use your writeup to bypass. Good job!

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Nice writeup! Wish i had this when i did mine lol

 

for anyone wary of tampering with the stock wiring might want to check out this technique: http://tech.240sxone.com/94/s14-how-to-build-an-amp-bypassdelete-harness/

(Adding the caps mentioned by slartibartfast will keep your tweeters if you have them)

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There is an alternative so you can use the factory amps, which worked on my 2000 Mustang GT like Slick Rick until I scratched up enough to replace the factory amps (and the factory amps weren't that bad on my 'Stang). I intend to do the same trick for my dual amp '93 pathy until I can do the whole shabang. The trick is, assuming you have dual front/rear RCA out on your new head unit to match the front/rear input setup on the factory wiring harness, is to get a dual RCA patch cable and cut it in half so you have two dual RCA cables terminating at the cut (you can probably see where I'm going with this already...). Instead of splicing the amplified outputs from the head unit into the harness adapter's inputs, strip back the insulation on the RCAs and solder those in instead to feed a line-level to the amp's inputs. Shield would go to "-", center conductor goes to "+", etc. Shrink tube it up and voila! Line level fed to the amp's inputs! And no cutting of the factory harness needed (you only need to jack with the little adapter pigtail that plugs into the factory harness). They do make those tunable resistors that go in-line to drop amplified signals to line-level, which was the first trick I used on the 'Stang. It melted from the heat, so I've since considered them a poor hack and fire hazard.

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  • 1 month later...

Late addendum: If you want a cheap workshop stereo, or a stereo for a cheap car, or whatever, don't chuck your amps after you take them out. They'll take an iPod (or anything else with a headphone jack and its own volume control) output and amplify it for car speakers. I made a workshop setup out of the rear amp, a couple little 1W speakers (only matched set I had laying around), a set of buggered earbuds (for the input cord), and drew 12v from a computer power supply. It sounds reasonably good and cost me nothing. B)

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